This application relates to optical polarization, and more specifically, to techniques and systems for controlling a state of polarization of an optical signal.
Various optical devices and systems can be sensitive to the state of polarization of an optical signal to be processed or transmitted. For example, certain coherent optical processing may require a match between the states of polarization of two separate optical beams when the two beams are superposed. For another example, a birefringent optical element may attenuate an optical signal differently when the polarization of the signal forms different angles with respect to a given principal axis of polarization of the element. An optical amplifier with a saturable gain medium may also produce a polarization-dependent gain when a polarization component with a high intensity saturates the gain medium and hence experiences an optical gain less than that of another, weaker polarization component. Furthermore, certain optical modulators may produce different modulation depths on optical signals with different polarizations. Semiconductor electro-absorption modulators and electro-optical modulators based on birefringent crystals such as lithium niobate are examples of such modulators. Hence, it is generally desirable to control the polarization of an optical signal in those and other polarization-sensitive devices and systems.
The polarization of an optical signal may not be static but dynamically vary with time in some optical systems due to various fluctuations or changes in some parts of the systems such as light sources, optical components, and optical transmission media. For example, some optical fibers may be birefringent to exhibit different refractive indices for different polarizations. Typical causes for this fiber birefringence include, among others, imperfect circular cores, and unbalanced stress in a fiber along different transverse directions. Fluctuations in local temperature and stress along a fiber line, therefore, may randomly change the axis of birefringence of the optical fiber at different locations. The polarization of light transmitting through such a fiber, therefore, may also fluctuate with time and cause polarization-mode dispersion (PMD) in optical signals with two orthogonal principal polarization states.
Accordingly, it may be desirable that a polarization control mechanism be dynamic so that it may change its control in response to any variation in the input polarization of light and therefore maintain or set the output polarization at a desired state. Some dynamic polarization control devices implement an adjustable polarization module that manipulates the polarization of light, and a polarization analyzer that measures any deviation of the actual output polarization from the polarization module from a desired output polarization. The adjustable polarization module may include multiple adjustable polarization elements, e.g., rotatable waveplates or adjustable fiber squeezers engaged to a fiber, to control the output polarization based on adjustable optical birefringence. A feedback control loop may be used to control the polarization elements in the adjustable polarization module to correct any variations in the input polarization based on the measured deviation from the polarization analyzer.
The present disclosure includes a control mechanism for dynamically controlling the multiple polarization elements in the adjustable polarization module by implementing two control mechanisms: a feed-forward control and a feedback control. In one embodiment, the feed-forward control measures the input polarization of the input signal and adjusts the multiple polarization elements to pre-determined settings for producing the desired output polarization. The feedback control adjusts the multiple polarization elements around the settings initially set by the feed-forward control to reduce the measured deviation of the output polarization of the adjustable polarization module. In another embodiment, the feed-forward control is engaged to control at least two polarization elements while the feedback control is engaged to control at least two polarization elements that are not engaged to be controlled by the feed-forward control.
To certain extent, the feed-forward control essentially provides a fast, coarse control of some or all of the polarization elements in response to the input polarization and the feedback control essentially fine tunes the settings of some or all of the polarization elements to reduce the deviation of the output polarization from a desired output polarization.